In computing systems that employ a mass storage device (such as a hard disk) for the storage of data objects (e.g., files), it is often the case that data objects stored on the mass storage device are “migrated” to backup media (e.g., a tape, a writeable optical disk, etc.) in order to make room for new data in mass storage. In performing the migration, one issue that arises is the effective use of backup media in a manner that efficiently exploits the physical properties of the migration system and of the media itself. Examples of such physical properties are: the availability of more than one drive to write plural media concurrently, the presence of a “juke box” (a device that stores a library of “near-line” media that can be mounted and dismounted on drives by a robot without human intervention), and the double-sided nature of certain media.
For example, when plural drives are available to read different media concurrently, it may be undesirable to use only a single medium because doing so fails to exploit the time efficiency that could be realized by using extra drives concurrently. However, allocation of too many media is also undesirable, because of the cost of additional media, as well as the additional time that it takes to mount and dismount numerous media onto and off of the drives. Other considerations that can affect efficiency are the different amount of time that it takes to mount “near-line” media (as compared with “off-line” media stored outside a juke box library), and the presence of double-sided media (which have the advantage that they can store more data on one medium than can be stored on a similar single-sided medium, but also have the disadvantage that one side of the medium is unavailable when the medium is mounted on a drive on the other side).
While migration systems exist that are capable of storing migrated data on media, they do not provide efficient uses of media, plural drives, juke boxes, and double-side properties in view of the considerations outlined above. The present invention overcomes the limitations and drawbacks of the prior art.